endo-Glycals (1,2-unsaturated sugars) are well recognized as versatile building blocks in preparing numerous biomolecules. In particular, Danishefsky's glycal assembly method offers a novel and efficient approach to synthesize Lewis blood group determinants, gangliosides, and tumor-associated antigens. See e.g., Williams et al., in Carbohydrates in Chemistry and Biology, Wiley-VCH Verlag GmbH: Weinheim, Germany, (2000) Volume 1, pp 61–92. 2,3-Unsaturated glycosides have been synthesized from endo-glycals through a Lewis acid-catalyzed allylic rearrangement (Ferrier reaction). They can be readily converted to bioactive products such as forskolin and cyclophellitol. See e.g., Henry et al., J. Org. Chem., (1994) 59:5128 and Fraser-Reid B., Ace. Chem. Res., (1996), 29:57. Recently, Michael addition to 2-nitrogalactal was applied for the synthesis of TN antigens, STN antigens, and other glycopeptides. See e.g., Winterfeld et al., Angew. Chem. Int. Ed. Engl., (2001) 40:2654.
Compared to endo-glycals, there have been few reports on the chemistry of exo-glycals because of their uncommon or tedious preparations.